Next-generation long-haul, fiber-optic communications systems are being designed to operate at 100 gigabits per second over distances of 1,000 kilometers or more. Coherent optical receivers have been proposed as an alternative to conventional direct detection receivers for high-speed, fiber-optic systems because, among other reasons, they recover the phase of optical electric fields. When in-phase (I) and quadrature (Q) components of an optical signal are known, exact equalization of linear channel impairments is possible in principle and the effects of nonlinear impairments may be reduced.
Time “skew” in a coherent optical communications system refers to timing misalignment between data sent in I and Q channels of the system. Ideally, high-speed data bits in I and Q arrive at a receiver synchronized. In practice, however, data in I may arrive slightly ahead or behind data in Q and the data are said to be “skewed”. Best receiver performance, e.g. lowest bit-error rate, is obtained when skew is minimized. However, before skew can be corrected, it must first be detected and measured.